Various metering devices of this type have been proposed before and reference may be made to a WO 90/10190A and WO00/75611A which disclose arrangements of this general type.
The GB-A-2377425 discloses a metering device of the type described above in which the valving arrangement comprises a spool valve having a spool sealingly slideable within a bore. A reciprocating mechanism is provided to drive the spool axially between two alternate positions in response to the shuttle reaching the initial position or the second position within the chamber. In one position the spool valve creates a fluid flow path for pressured liquid from a fluid flow inlet duct to one end of the chamber and also creates a fluid flow path from the other end of the chamber to a fluid flow outlet duct. In the second position of the spool a fluid flow path is created from a fluid flow inlet duct to the other end of the chamber and from the said one end of the chamber to the fluid flow outlet duct.
With a reciprocating movement the spool valve is subjected to many changes of momentum, which can cause wear in the drive mechanism.